In extruders of the screw type, a plastic, extrudable material to be processed is urged into a long passageway within a barrel in which a snugly fitting screw rotates, forcing the extrudable material toward a shaping die and heating the extrudable material by shearing it between the screw and passageway wall. The resulting high pressures, temperatures, and stresses within the extruder passageway are an essential part of processing the extrudable material but they are also a source of wear on the extruder. Abrasive wear can occur when viscous extrudable materials having abrasive components which resist shearing actions flow under high pressures across screw and passageway wall surfaces. Adhesive wear can occur when the screw and extruder passageway wall momentarily comes into contact with the extruder and forms a small weld which then immediately breaks away, leaving small, irregular pits or cracks in the screw and passageway wall surfaces. Corrosive wear can occur as a result of corrosive attack on passageway wall and screw surfaces by the material being extruded, accelerated by the high temperatures and pressures inside the extruder passageway.
Wear can have a profound effect on the efficiency and effectiveness of the extruder operations. As the passageway wall and the screw surfaces wear down, the flow of extrudable material therebetween changes, causing a reduction in the shear stresses, temperatures, and pressures within the extruder passageway resulting in inadequately processed extrudate. Also, the wiping of extrudable material from the passageway wall by rotation of the snugly fitting screw is reduced, leaving a coating of extrudable material on the passageway wall which acts as a barrier to heat transfer between the extrudable material and the barrel, thereby causing ineffective heating or cooling of the extrudable material.
Replacement of the barrel or relining of the interior of the barrel (e.g. the passageway wall) is necessary to restore effective extruder operation. Replacement of the barrel is generally expensive, especially for a large extruder. Therefore, methods have been devised to reline the interior of the barrel by providing a new passageway wall. The most commonly practiced relining methods include slip-fitting a sleeve liner into the barrel or shrink-fitting a sleeve liner into the barrel bore.
Slip-fit and shrink-fit methods both use elongated cylindrical sleeve liners having an interior passageway therethrough, the surface of which is coated with a wear and/or corrosion resistant material. Generally, sleeve liners are made by a process of high temperature spin coating in which the annular space within a section of rigid steel tubing is charged with a metal alloy and placed in a furnace. The tubing is then heated and spun along its cylindrical axis, depositing a hard metal alloy coating substantially uniformly along its interior wall. The coated wall can then be machined to form the required configuration for an extruder passageway which will fit snugly about the screw.
In the slip-fit process, a sleeve liner thus coated, is axially mated within the barrel. The barrel is first prepared by machining away the interior surfaces to create a cylindrical bore (of exacting diameter and straightness) within the barrel. The interior bore diameter is slightly larger than the exterior diameter of the sleeve liner to be slip-fit therein. The sleeve liner is then axially mated within the barrel bore by pressing it into the barrel bore.
The slip-fit method, however, oftentimes results in problems of inadequate heat transfer from the barrel through the sleeve liner and in slippage of the sleeve liner within the barrel bore during operation. These problems arise because of the difficulty of making a long straight bore within the barrel, i.e., some random deviation from straightness is inevitable. Therefore, in order to assure a fit with the straight sleeve liner, the barrel bore diameter must be made slightly larger than the exterior diameter of the sleeve liner, causing many areas along the mating surfaces between the barrel and sleeve liner to have loose contact or no contact at all. As a result of this lack of contiguous contact between the mating surfaces, heat transfer between the liner and the heating or cooling elements located on the barrel is impaired and the extrudable material is not adequately heated or cooled during processing. Also as a result of the loose fit, the sleeve liner can sometimes move axially or rotationally within the barrel bore under the forces generated during extruder operation, causing misalignment of the barrel and sleeve liner.
Modifications to the slip-fit method have been proposed to correct problems of heat transfer and sleeve liner movement. A patent issued to Seufert, U.S. Pat. No. 3,900,188, discloses a method of slip-fitting a cylindrical sleeve liner and securing it within the barrel by applying a hardening filler material between the barrel bore wall and the sleeve liner exterior surface, but that method has the disadvantage of further reducing heat transfer. Also, it is difficult to work with a hardening filler during insertion and removal of the sleeve liner.
Other modifications of the slip-fit process include using a sleeve liner having multiple segments which are shorter and thus more easily pushed into a slightly crooked barrel bore. The cross-sectional diameter of the barrel bore can therefore be machined to fit more closely within the barrel bore to improve heat transfer and retard sleeve liner movement. Those sleeve liner segments, however, have small gaps at their points of abutment within the barrel bore. Corrosion is caused on the interior surface of the barrel bore and on the sleeve liner by extrudable material which is forced through the gaps by the high pressures inside the passageway.
In the shrink-fit method, the sleeve liner is radially mated within the extruder barrel. The barrel is first prepared, in a manner similar to the slip-fit method, by machining away the passageway wall to remove the surface material and to create a cylindrical bore of exacting diameter and straightness within the barrel. The barrel bore is slightly smaller in diameter than the exterior diameter of the sleeve liner. The barrel is then heated to expand the barrel bore and while the barrel is still hot, the cooler sleeve liner, having an interior coating of wear-resistant material therein, is rapidly inserted into the bore. As the barrel cools, the barrel bore contracts and radially mates with the sleeve liner to form a tight compressive fit between the sleeve liner and the barrel.
The shrink-fit method, however, presents problems of excessive compression on the wear resistant coating, difficult removal for replacement, and difficult installation during replacement. The tight shrink-fit, accomplished by compression of the sleeve liner by the barrel, also compresses the wear resistant metal alloy coating. Since the coating tends to be hard and brittle, the barrel compression, combined with the high shearing pressures applied to the coating during extruder operation, causes the coating to flake off of the passageway wall. The compressive fit of the shrink-fit method also requires machining operations to remove a worn sleeve liner prior to replacing it, a process much more difficult than removal of a worn sleeve liner by simple pressing or driving. Finally, the installation of a shrink-fit sleeve liner within the barrel bore is a difficult process requiring exact timing and control to prevent the sleeve liner from mating at the wrong point within the barrel bore, and also requiring a substantial investment in special equipment not usually found in the machine shop of a production facility.
A modification to the shrink-fit process that aids in removal of a worn sleeve liner, but does not address other problems, was disclosed in the patent issued to Jerpbak, U.S. Pat. No. 4,133,460. The sleeve liner disclosed therein has exterior grooves which accommodate explosive charges between the sleeve liner and housing so that upon detonation of the explosive charges the sleeve liner break up for easy removal.
Outside of the screw extruder art, a variety of methods and apparatuses have been disclosed relating to sleeve type liners, but none of them teach an embodiment applicable to the relining of extruder barrels. In a patent issued to Wilson, U.S. Pat. No. 2,832,653, a substantially cylindrical sleeve liner was mated within a substantially tapering cylinder housing of a fluid pump to create a cavity between the sleeve liner and cylinder housing. The cavity can then be filled with a high pressure fluid to facilitate release of the sleeve liner from the cylinder housing. The Wilson patent does not teach the use of a tapered liner insert whose cross-sectional diameter closely matches the interior housing bore diameter, to provide contiguous contact between the surfaces and, thus permitting the heat transfer between sleeve liner and housing, which is required by a screw type extruder. In a patent issued to Boulton, U.S. Pat. No. 2,500,340, a sleeve liner is used to resurface the piston chambers of automotive master brake cylinders. The sleeve liner disclosed therein employed a maleable metal to be pressed into the piston chamber which would deform only at its ends to properly seat the sleeve liner within the piston chamber. If it were pressed into a bore as elongated as the bore within an extruder barrel, such a maleable metal would severely deform during pressing. Deformation destroys the snug fitting required in extruder barrel reliners, making the liner unuseable if the deformation is too severe.
It is therefore an object of the present invention to provide a relinable extruder barrel assembly having a rigid elongated housing and also having a rigid elongated sleeve liner closely fitting within the housing, wherein the sleeve liner is easily removable from the housing, and replacable using a slip-fit method.
Another object is to provide a relinable extruder barrel assembly having a housing and a sleeve liner that resists movement within the housing during extruder operation.
Another object is to provide a relinable extruder barrel assembly having a housing and a sleeve liner wherein the sleeve liner has a wear and/or corrosion resistant interior surface defining a passageway therethrough. The wear resistant coating surface is uniformly centered around the screw residing in the passageway, and resists flaking during slip-fitting and during extruder operation.
Another object is to provide a relinable extruder barrel assembly comprising a housing bore and a sleeve liner which are shaped to provide a substantially contiguous line-to-line fit therebetween, and therefore a uniform heat transfer between the housing and the sleeve liner is provided.
Another object is to provide a relinable extruder barrel assembly having a housing and a sleeve liner that can be readily relined at production plant level from a stock of replacement sleeve liners.
Another object is to provide a method for relining extruder barrels with a sleeve liner which may be easily removed and replaced using a slip-fit method.